1. The Field of the Invention
The present invention concerns a method and apparatus which will correct for the effect of small amounts of gas present during dielectric measurement of oil and water mixtures by using the slip velocity of the free gas.
2. The Prior Art
In a typical oil field application, water cut determination is performed by measuring the composite dielectric of a flowing mixture of oil and water. An example of this can be found in U.S. Pat. No. 4,873,648, the disclosure of which is incorporated herein by reference. Because of the large difference in the dielectric constants of water and oil, this is a sensitive method of water cut determination and is easily performed with such conventional instruments as capacitance probes. This measurement method requires only that the flow be well mixed, the oil in continuous phase, and assumes that no free gas is present. The limitation of "no free gas" is necessary because typical dielectric instruments perform only a single dielectric measurement while the variables determining the composite dielectric constant comprise both the water fraction as well as the gas fraction.
The prerequisite that there be no free gas is often violated, at least to a small degree, in most separator systems dealing with live crude oil. The present invention proposes a method and apparatus to reduce errors in water cut measurements due to free gas as well as obtaining a measure of free gas present.